1. Field of the Invention
This invention pertains to the field of classification of ore particles for a subsequent concentration process.
2. Description of the Prior Art
An ore is a mixture of valuable ore minerals and less valuable gangue minerals. These minerals appear as crystals or grains in the solid ore body. Utilization of the ore is almost always combined with a concentration process or an ore dressing process which aims at recovering the valuable ore minerals from the ore.
In the concentration of an ore or a similar mixture of different substances, the first step usually is to crush or grind the mixture to particles. Grinding results in a collection of different types of particles, e.g., pure monogranular particles consisting of a single substance, and polygranular particles, which can consist of up to as many different substances as there are different mineral grains included in the particles.
The more the ore is ground, the finer are the particles, also in relation to the size of the crystals, and the higher is the degree of liberation or of difference in composition between polygranular mixed particles. Both these properties regulate the grade of the concentrate and the feasibility of the concentration process.
Grinding or crushing is an expensive process, and particularly expensive is the production of small particle sizes.
Additionally, very fine particles, e.g., slime, are also to be avoided because they disturb the concentration process. To diminish the production of slime, grinding is carried out in steps so that the ore is brought to a mill and ground, commonly in water, to a coarse size such that only a modest liberation is achieved. The ground material then goes to a dividing device that produces a fine product which has the desired liberation for the concentration process and a coarse product which is recirculated back to the mill, reground and then again brought to the dividing device. The fine product from this device has less slime than the ore would have had if ground directly to the desired liberation and the product has also been cheaper to make.
In most cases, classifiers are used as such a dividing device and operate on the principle that particles that are setting slowly, i.e. smaller and/or higher particles, are brought to the concentration process while particles falling more rapidly, i.e. larger and/or heavier particles, are returned to repeated grinding. Small, pure, heavy particles fall with the same velocity as larger, light mixed particles. The concentration process, therefore must operate with a particle mixture where the heavier material is in a more ground state than the lighter material. Pure, heavy particles are returned to the grinding device and come back often as a too finely ground slurry while light mixed particles, which would have needed to be additionally ground, are discharged to be concentrated.
If, instead, this dividing device were a conventional screen, the separation would take place according to particle size only, regardless of particle density. However, conventional screens are not sufficiently reliable at the fine size ranges in question; they are commonly used for relatively coarse separations, e.g., at 5 mm or more and are, therefore, rare in this context.